Doc: document possible need to raise kernel's somaxconn limit.

On fast machines, it's possible for applications such as pgbench
to issue connection requests so quickly that the postmaster's
listen queue overflows in the kernel, resulting in unexpected
failures (with not-very-helpful error messages).  Most modern OSes
allow the queue size to be increased, so document how to do that.

Per report from Kevin McKibbin.

Discussion: https://postgr.es/m/CADc_NKg2d+oZY9mg4DdQdoUcGzN2kOYXBu-3--RW_hEe0tUV=g@mail.gmail.com

doc/src/sgml/runtime.sgml

@@ -1299,6 +1299,22 @@ default:\
     linkend="guc-max-files-per-process"/> configuration parameter to
     limit the consumption of open files.
    </para>
+
+   <para>
+    Another kernel limit that may be of concern when supporting large
+    numbers of client connections is the maximum socket connection queue
+    length.  If more than that many connection requests arrive within a very
+    short period, some may get rejected before the postmaster can service
+    the requests, with those clients receiving unhelpful connection failure
+    errors such as <quote>Resource temporarily unavailable</quote> or
+    <quote>Connection refused</quote>.  The default queue length limit is 128
+    on many platforms.  To raise it, adjust the appropriate kernel parameter
+    via <application>sysctl</application>, then restart the postmaster.
+    The parameter is variously named <varname>net.core.somaxconn</varname>
+    on Linux, <varname>kern.ipc.soacceptqueue</varname> on newer FreeBSD,
+    and <varname>kern.ipc.somaxconn</varname> on macOS and other BSD
+    variants.
+   </para>
   </sect2>
 
   <sect2 id="linux-memory-overcommit">